As described for example in U.S. Pat. No. 7,008,979; fog formation occurs under conditions of high humidity and high temperature or at interfacial boundaries where there is a large temperature and humidity difference. Coatings which reportedly reduce the tendency for surfaces to “fog up” (i.e., anti-fogging coatings) have been suggested.
In order to prevent this fogging, it is known to use various surface active agents to provide anti-fog properties to articles by adding hydrophilic agents to polymeric compositions in order to impart anti-fog properties. For example, anti-fog coating compositions for transparent surfaces which include a three-dimensional cross-linked polyurethane having a free surface active agent disposed within open domains in its cross-linked structure have been suggested. The coating compositions are prepared by reacting isocyanates with polyfunctional polyols to obtain a polyurethane, and subsequently contacting the thus prepared polyurethane with a hydrophilic surface-active agent in order to diffuse molecules of the surface-active agent into the interior of the coating. (See for example U.S. Pat. Nos. 4,551,484 and 4,609,688 to Radisch et al.) The surface-active agent, however, is not chemically reacted into the polyurethane, but is instead physically disposed within the polymeric structure. As such, the cured coating is susceptible to undesirable leaching and erosion of the surfactant, thereby decreasing the anti-fog properties of the coating composition. Similarly, EP 0399441 to Chen et al. discloses an antifog coating composition including a highly crosslinked acrylic coating or a nanosilica-containing hardcoat and a free-surfactant. WO 1997/043668 to Ko et al. discloses articles including a coating on the surface of a substrate, in which the coating contains a fluoropolymer and a fluororchemical surfactant.
It has also been proposed to react surface active agents into a polymeric coating composition in order to impart anti-fog properties to the coating composition. For example, the addition of sulfonated “resins” to polyurethanes in order to prepare coatings with various properties including anti-fog characteristics have been suggested. The resins are prepared from diols or diamines reacted with di-carboxylic acid esters, followed by sulfonation of double bonds or quarternization of amines. The resins are intended to increase the hydrophilic character and water absorption of the polyurethane coatings by reacting into the polyurethane backbone in an end-to-end fashion, rather than as pendent groups. Such resins which react in an end-to-end fashion, as opposed to remaining pendant at the end of the polyurethane chain, cannot provide for a clear delineation of hydrophilic and hydrophobic groups and in this respect do not behave as surfactants, i.e., they do not provide cooperation between distinct hydrophilic and hydrophobic portions to reduce interfacial tension. (See for example U.S. Pat. No. 3,822,238 to Blair et al.) In addition, U.S. Pat. No. 7,776,937 to Kato et al. discloses a photocurable two-part coating composition including a multifunctional acryloyl-containing urethane compound and a multifunctional acrylate having at least two free-hydroxyl groups. Langmuir vol. 28, issue 49, pp. 17193-17201 (2012) to Chang et al. discloses a two layered construction having an organic-inorganic composite acrylic hardcoat as a supporting layer at the bottom, and an anti-fog coating layer containing an acryloyl functional surfactant copolymer on the supporting layer. However, these compositions do not provide permanent fog resistance properties, i.e. fog resistant properties which last after repeated washings or extended soaking in water, nor are they effective for more than a few hours of use.